Transmitter output stage for a two-wire bus

ABSTRACT

The invention relates to a transmitter output stage for a two-wire bus, which output stage imposes equal but opposed currents on the two wires ( 7, 8 ) of the bus and has a first voltage source ( 5 ) for supplying voltage, a second voltage source ( 6 ) for controlling the equal but opposed currents and for generating data bits on the bus wires ( 7, 8 ), and two PNP transistors ( 1, 2 ) whose bases are driven by the second voltage source ( 6 ) and which both generate equal collector currents of which one ( 11 ) is fed to the first bus wire ( 7 ) and a second (IT 1 ) is fed to an input of a current mirror circuit ( 3, 4 ) that, at the output end, imposes on the second bus wire ( 8 ) a current of equal size but opposite sign to the current ( 11 ) fed to the first bus wire ( 7 ).

The invention relates to a transmitter output stage for a two-wire bus.

In two-wire bus systems, e.g. the CAN bus, use is made of so-calledcommon mode chokes. These are provided separately from the transmitteroutput stages and the purpose they serve is to reduce theelectromagnetic radiation from the bus system. This is achieved byvirtue of the fact that the common mode chokes impose currents of equalsize but opposite sign on the two wires of the bus system.

The common mode chokes represent an additional cost for a bus system ofthis kind because they have to be provided as separate pieces ofcircuitry.

It is an object of the invention to provide a transmitter output stagethat itself minimizes the electromagnetic radiation from the bus bymeans of equal but opposed currents, thus enabling common mode chokes tobe dispensed with.

This object is achieved in accordance with the invention by means of thefeatures of claim 1:

a transmitter output stage for a two-wire bus, which output stageimposes equal but opposed currents on the two wires (7, 8) of the busand has a first voltage source (5) for supplying voltage, a secondvoltage or current source (6) for controlling the equal but opposedcurrents and for generating data bits on the bus wires (7, 8), and twofirst transistors (1, 2) whose bases are driven by the second voltagesource (6) and which both generate equal collector currents of which one(I₁) is fed to the first bus wire (7) and a second (I_(T1)) is fed to aninput of a current mirror circuit (3, 4) that, at the output end,imposes on the second bus wire (8) a current (I₂) of equal value butopposite sign to the current (I₁) fed to the first bus wire (7).

A transmitter output stage of this kind has a first voltage source thatis used to supply current or voltage to the output stage. By means of asecond voltage source or current source, a drive is provided for thebases of two first transistors that are so designed that they generateequal collector currents. A first one of these collector currents is fedto a first wire of the two-wire bus. The current of equal size generatedby the second transistor is fed to a current mirror circuit and thelatter produces, at the output end, a current of equal value butopposite sign that is fed to the second wire of the two-wire bus.

What is achieved in this way is that currents of the same magnitude butdifferent sign are fed to the two bus wires in the desired manner, thusminimizing the electromagnetic radiation from the two wires of the bussystem.

Hence, there is no longer any need for additional circuits, such as thecommon mode chokes known from the prior art, for example, to allow thiseffect to be achieved.

Rather, the transmitter output stage, under the control of the secondvoltage source, at all times ensures that equal but opposite currentsare fed to the two wires of the bus system. This is true both when thetransmitter output stage is operating off-load, i.e. when no data isbeing fed to the data bus, and when it is in the active mode, namelywhen data is being fed on. These two modes (provision may also be madefor more than two modes) may be selected by selecting the value of thevoltage that is supplied by the second voltage source.

Hence, what is available in the final analysis is a very simple circuitfor a transmitter output stage that is suitable both for transmittingdata and for minimizing the electromagnetic radiation from the two-wirebus.

Because of the different polarities of the two currents, two PNPtransistors may be provided as the two first transistors that supplyequal currents, as is provided for in an embodiment of the inventionthat is claimed in claim 2. The current mirror that supplies a currentof equal magnitude but opposite sign as compared with the current fed tothe first bus wire is advantageously constructed from two NPNtransistors.

In a further embodiment of the invention, detailed in claim 3, thesecond voltage source that controls the size of the currents that arefed to the two wires of the two-wire bus may supply a plurality ofvoltages of different values that initiate, for example, a quiescentstate for the data bus and one or more active states on the data bus.

As is provided for in a further embodiment of the invention that isclaimed in claim 4, the data bus may, for example, be a CAN bus forwhich, because it is used in vehicles, it is particularly important forthe electromagnetic radiation to be minimized.

These and other aspects of the invention are apparent from and will beelucidated with reference to an embodiment described hereinafter andwith reference to the sole FIGURE of the drawings.

The FIGURE shows, in the form of a circuit diagram, a transmitter outputstage according to the invention having a first voltage source 5 thatsupplies the circuit of the transmitter output stage with voltage.

Also provided is a second voltage source 6 that drives the bases of afirst PNP transistor 1 and a second PNP transistor 2. The emitters ofthese two PNP transistors 1 and 2 are connected to the positive pole ofthe first voltage source 5.

The two PNP transistors 1 and 2 are so designed that they generatecollector currents of equal value. The collector current of the PNPtransistor 2, which is identified in the FIGURE as I₁, is fed to a firstbus wire 7 of a two-wire bus system that is not otherwise represented inthe FIGURE.

The aim is to feed a current of equal value but opposite sign to asecond bus wire 8 of the two-wire bus.

For this purpose, use is made of the collector current of the first PNPtransistor 1, which is identified in the FIGURE as I_(T1) and which isof the same value and sign as the current I₁.

This current I_(T1) is fed to a current mirror circuit that isconstructed from two NPN transistors 3 and 4. The emitters of the twoNPN transistors 3 and 4 are connected to the negative pole of the firstvoltage source 5. The input transistor 3 of the current mirror circuitis connected as a diode, i.e. its base and collector are connectedtogether. The current I_(T1) from the first PNP transistor 1 is fed tothe collector of the input transistor 3 of the current mirror circuit.Hence, the output transistor 4 of the current mirror circuit generateson its collector a current of equal value but opposite sign. Thiscurrent is identified in the FIGURE as I₂.

Hence, current I₂ is precisely the desired current that, compared withthe current I₁, is of equal magnitude but opposite sign. This current isfed to the second bus wire 8.

This ensures that currents of opposite sign but equal magnitude flow onthe two bus wires 7 and 8.

For its part, the voltage source 6 may be controlled, i.e. it maydeliver a plurality of different voltage values that may, for example,be used to produce various states on the data bus, or rather on its buswires 7 and 8.

Even when there are different voltage values from the voltage source 6it is ensured, provided these values are equal to or lower than thevoltage values from the first voltage source 5, that equal but opposingcurrents are always imposed on the bus wires 7 and 8 by the transmitteroutput stage, both when the latter is in a quiescent state and is nottransmitting any data and when it is in an active state in which data isbeing fed to the two bus wires 7 and 8 of the two-wire bus.

Hence, the object stated above is achieved in any condition ofoperation. This is done with a relatively simple construction for thetransmitter output stage and without any additional circuits beingrequired.

The transistors at the bus end, i.e. the transistors 2 and 4, may bereplicated, that is to say that more than one such transistor may beprovided connected in parallel, in order to obtain a larger outputcurrent.

In place of the voltage source 6, it is also possible for a currentsource to be provided that feeds a current to the bases of thetransistors and whose other pole is connected to the input of thecurrent mirror circuit. The transistors 1 to 4 may also take the form offield-effect transistors.

If provision is made for the currents to be regulated by appropriatelysetting the voltage from the second voltage source 6, such regulationshould be oriented to the voltage on the first bus wire 7, given that adesired voltage is generally preset for the first bus wire. For thispurpose, provision may, for example, be made for suitable feedback ofthe voltage on the first bus wire 7 to the voltage source 6.

1. A transmitter output stage for a two-wire bus, which output stageimposes equal but opposed currents on the two wires (7, 8) of the busand has a first voltage source (5) for supplying voltage, a secondvoltage or current source (6) for controlling the equal but opposedcurrents and for generating data bits on the bus wires (7, 8), and twofirst transistors (1, 2) whose bases are driven by the second voltagesource (6) and which both generate equal collector currents of which one(I₁) is fed to the first bus wire (7) and a second (I_(T1)) is fed to aninput of a current mirror circuit (3, 4) that, at the output end,imposes on the second bus wire (8) a current (12) that is of equal valuebut opposite sign to the current (I₁) fed to the first bus wire (7). 2.A transmitter output stage as claimed in claim 1, characterized in thatthe two first transistors are in the form of PNP transistors (1, 2) andtheir emitters are connected to the positive pole of the first voltagesource (5), in that a first one (1) of the two PNP transistors (1, 2)has its collector connected to the collector and the base of an NPNinput transistor (3) in the current mirror circuit, which circuit has anNPN output transistor (4) whose base is connected to the base of theinput transistor (3), the emitters of both the transistors (3, 4) in thecurrent mirror circuit being connected to the negative pole of the firstvoltage source (5).
 3. A transmitter output stage as claimed in claim 1,characterized in that the second voltage source (6) is able to supply aplurality of different voltage values that initiate a quiescent statefor the data bus and at least one active state on the data bus.
 4. Atransmitter output stage as claimed in claim 1, characterized in thatthe data bus is a CAN bus and in that the second voltage source (6) isable to supply two different voltage values, one of which initiates aquiescent state for the data bus while the other initiates an activestate on the data bus.